(a) Field of the Invention:
The present invention relates to an apparatus for performing plasma chemical vapor deposition, and more particularly it relates to the electrode structure for use in plasma vapor deposition apparatuses of the capacitance coupled type.
(b) Description of the Prior Art:
The so-called plasma chemical vapor deposition technique which will hereinafter be called simply "plasma CVD" technique comprises: introducing a gaseous reaction material for chemical reaction into an appropriate closed but evacuatable chamber and developing a glow discharge in said chamber to cause reaction between the enclosed material to deposit the reaction product on the smooth surface of a substrate housed in said chamber. This technique is indispensable at the present stage of art as a widely utilized process in the manufacture of semiconductor devices and various other electric as well as electronic parts such as the formation of thin insulating films of SiO.sub.2 or Si.sub.3 N.sub.4, or thin semiconductor layers of, for example, amorphous silicon (a-Si). The apparatuses employed for performing such CVD process are, basically, divided roughly into the induction coupled type and the capacitance coupled type. The former is operated in such a way that the glow discharge is induced by a radio frequency coil (RF coil) which is wound around the reaction chamber. The latter is performed so that the glow discharge is induced by a radio frequency power applied across the electrodes provided within the chamber. FIG. 1 shows the basic arrangement of a conventional example of the CVD apparatus of the induction coupled type. In FIG. 1, reference numeral 1 represents a closed type reaction chamber; 2 an RF coil wound around the reaction chamber 1; 3 a tank for storing a gaseous reaction material such as silane (SiH.sub.4) gas requiring a reaction; 4 a conduit pipe for introducing, into the reaction chamber 1, the gaseous reaction material contained in the storage tank 3; 5 a drum-like substrate rotatably supported within the reaction chamber 1; symbol M a motor for rotating the drum-like substrate; symbol RF a radio frequency power supply connected to the RF coil 2; and symbol RP an evacuation pump. This conventional CVD apparatus is utilitzed in such a way that a glow discharge is caused while introducing gaseous reaction material such as SiH.sub.4 gas into the reaction vessel 1 via the conduit pipe 4, and concurrently therewith the drum-like substrate 5 is rotated by the motor M, whereby an a-Si film is deposited as a thin layer of the surface of the drum-like substrate 5. In this conventional apparatus, the structure per se of the reaction chamber 1 is relatively simple. However, this known apparatus has the disadvantages in that it is difficult to make uniformal the thickness of the film formed, by deposition, on the surface of the substrate 5 and that there is a difficulty in controlling the film-forming rate. Description has been made above with respect to the instance wherein a film serving as photo-receptor is manufactured. It should be noted that various problems are entailed also in the manufacture of other types of devices.
In contrast thereto, the CVD apparatus of the capacitance coupled type has recently been tended to be adopted, in place of the induction coupled type CVD apparatus, as being an apparatus capable of eliminating the above-mentioned inconveniences and drawbacks of the induction coupled type CVD apparatus.
FIG. 2 shows a basic arrangement of a conventional example of the capacitance coupled type CVD apparatus. In FIG. 2, like reference numerals and symbols are assigned to those component parts which are practically the same as those in FIG. 1. That is, reference numeral 6 represents a hollow tubular RF electrode having a number of gas-ejecting perforations 7 surrounding the drum-like substrate which concurrently serves as an electrode; and 8 a shielding plate covering the entire external circumstance of the hollow tubular electrode 7 to insure that the glow discharge is restricted between the drum-like electrode 5 and its opposing electrode 7. This conventional apparatus having the above-mentioned arrangement is operated in such a way that, while introducing, for example, SiH.sub.4 gas into the reaction vessel 1 via the conduit pipe 4, a glow discharge is induced across the electrodes 5 and 7, to thereby cause deposition of an a-Si film onto the surface of the drum-like electrode 5. With a capacitance coupled type CVD apparatus of the prior art having such an arrangement as mentioned above, it is possible to make uniformal the thickness of the entire film which is produced by deposition, and also it is easy to control the film-forming rate. In order to produce a film having a good function and ability with a high productivity, however, this conventional apparatus still possesses various problems to be solved.
These drawbacks and inconveniences encountered in the conventional CVD apparatuses will hereunder be described with respect to the instance wherein the a-Si film thus produced is utilized as a photo-receptor.
In order to improve the function and ability of the a-Si film in general, it is necessary to give considerations to such points as mentioned below. They are: (a) reducing the flow rate and the mol ratio of the gas which is introduced into the chamber to relatively lower the film-forming rate; (b) using RF power of as small a density as possible; (c) using a low-pressure gas of a level lower than 1 Torr during the glow discharge; and (d) set the substrate at an appropriate temperature. This means that the factors such as the flow rate of SiH.sub.4 gas, the RF power value and the distance between the electrodes are to be so selected as to provide adequate conditions.
In such devices as solar batteries and thin-film transistors using a-Si films, it should be noted that the a-Si films used therein may have a film thickness of about 1 micrometer or smaller, so that, even when importance is attached to the ability and function of the produced film, the formation of the film does not consume so much time when attention is given to the fact that the film-formation rate by the conventional plasma CVD apparatus is 1.about.2 .mu.m/hour, and thus it will be appreciated that the film thickness does not greatly affect the productivity. However, in order to use an a-Si film to serve as a photo-receptor, the film requires usually a thickness ranging from 10 to 50 micrometers. Thus, the acquisition of a photo-receptor having a required thickness will take several to several ten hours according to this conventional apparatus, which is indeed impractical. Therefore, in order to form an a-Si film in a ralatively short period of time, i.e. in order to operate the manufacturing apparatus within a practical length of time by improving the film-forming rate, it will be necessary to increase the flow rate and the mol ratio of SiH.sub.4 gas, and to elevate the RF power value as well as the pressure of gas which is applied, contrary to the instance performed by the conventional apparatus. However, as will be described later, such a contrary manner of operation as mentioned above has introduced various practical drawbacks in the conventional apparatuses such as lowering the quality of the film or the incapability of successive formation of films on a plurality of drums. Even with the apparatus having such an improved electrode structure as shown in FIG. 2 to form a good-quality film at a relatively high film formation speed, there still are present such problems as mentioned above. Thus, it has been impossible to obtain an a-Si film having a good function and ability and having a uniform required film thickness in one cycle of plasma CVD operation.
In the effort of improving the film-forming rate or in case the apparatus is used for an extended period of time in the conventional plasma CVD apparatuses, the first fatal reason for bringing about lowering of the function and ability of the a-Si film which is produced is as mentioned below. The decomposition of a reaction material such as SiH.sub.4 caused by a glow discharge takes place in such a manner as follows. That is, decomposition of the material starts at the stage that a gas is introduced into the region in which a glow discharge is being developed. This decomposition is accelerated as the gas advances through this region, and radicals (herein, this term collectively means those activated Si atoms and neutral Si-H or SiH.sub.2 which have been activated by decomposition) will progressively augment in their amount. As these radicals increase, they will come to collide with each other in the gaseous atmosphere and will coaggulate, growing into fine particles of a-Si and other substances. These respective particles will further coaggulate together into respective fine powders. An a-Si film, thus, is formed not only on the surface of the substrate alone, but also will be formed over all other areas of the glow discharge region. As a result, in the vicinity of the gas-ejection orifices 6 of the opposing electrode 7, deposits of a-Si and other substances will adhere, in a relatively tough state, onto the surface of this electrode. On the other hand, at sites of the electrode located remote from the ejection orifices, powdery substances will deposit. In the intermediate areas located therebetween, substances which are in a condition intermediate of the above-mentioned two types of conditions will deposit. Such phenomena take place, as a matter of fact, more intensively for a greater flow rate of gas and for a higher pressure of reaction which are employed, and for a broader region of glow discharge, encompassing a wide range of areas within the reaction chamber. This deposition phenomenon as mentioned above takes place most intensively on the surface of the opposing electrode. Accordingly, if the film-forming rate is increased or if the plasma CVD is carried out for an extended period of time, such deposition of unnecessary substances will augment accordingly. At the time the amount of this deposition becomes critically large, the deposits will begin to come off from, for example, the surface of the opposing electrode due to, for example, stress-strain caused by the difference in thermal expansion of the substances, and these freed particles will scatter in the form of the so-called fine flakes, and they will hit the surface of the drum and will damage the film itself which is formed thereon. Also, there would arise the phenomenon that very fine powder which scatters along with the above-mentioned fine flakes, or that those fine particles produced in the gaseous atmosphere will become mingled into the aimed a-Si film formed on the surface of the drum, and will lower the function and ability of this formed a-Si film.
In addition, it should be noted that conventional plasma CVD apparatuses are arranged so that the drum 5 which serves as the substrate is used concurrently as an electrode, and that there is provided a rigid metal electrode 6 made of, for example, stainless steel having a number of gas ejection orifices 7 and this electrode is arranged so as to surround the drum 5 to face it. Besides, a metal plate having a rigidity is usually provided around the external circumference of the metal electrode 6 to shield it. This shielding plate 8 is intended to prevent the glow discharge from expanding widely to other regions than the space across the two electrodes when the reaction gas is introduced at a low pressure level. The presence of this shielding plate 8, while restricting the glow discharge to occur relatively in the space between the two electrodes, will naturally tend to cause the glow discharge to be contained in a restricted spatial region. Thus, there tends to occur the stagnation of the introduced gas, and it has been impossible to avoid the development of the abovesaid flakes and the acceleration of their development.
As discussed above, in the conventional plasma CVD apparatuses mentioned, it has been difficult to substantially increase the film-forming rate. Even if this rate is ever increased, the development of flakes and fine powder of substances has contributed to the lowering of the ability and function of the film produced. Also, the conventional apparatuses have the further drawback that, owing to the development of flakes and fine powder, the plasma CVD could not be carried out continuously for an extended period of time.